Method of lowering the viscosity of hydrolyzed cellulose esters and product



Patented Apr. 8, 1 952 HYDROLYZED CELLULOSE PRODUCT ESTERS AND Carl J. Malm and Leo J. Tanghe, Rochester, N. Y.,

assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey No Drawing. Application April 30, 1949,

Serial No. 90,766

5 Claims.

soluble cellulose acetates often exhibit high acetoneviscosities due to the alkaline earth metal salts which may be present therein. This phenomena was described in U. S. Patent No. 2,126,489 of Malm and that patent described and claimed a method of lowering the viscosity of the cellulose ester by washing the ester with a series of distilled water Washes. Apparently'this treatment removed the greater portion of the calcium andmagnesium salts, thus lowering the acetone viscosity of the esters without breaking down the cellulose itself. As a consequence, films or filaments of good tensile strength can be prepared from esters which do not exhibit extraordinarily high acetone viscosities.

One object of our invention is to lower the viscosity of hydrolyzed cellulose esters without .breaking down the cellulose itself, thus making possible'high concentrations of esters of undegraded cellulose in organic solvents. Another object of our invention is to provide a cellulose ester having a low viscosity in acetone considering the intrinsic viscosity of the cellulose ester. Other objects of our invention will appear herein.

In the manufacture of cellulose acetate yarn the cellulose acetate is dissolved in a volatile solvent, usually acetone, and is converted into fyarn'by extruding this solution through a spinnerette into an evaporative atmosphere. Ordinarily the limit of the solubility of the cellulose ester employed for making yarn in the acetone is such that a 25% concentration thereof is about the limit for obtaining a satisfactory spinning solution. If the ester has a sufiiciently low viscosity that a greater proportion of the cellulose ester can be dissolved therein without form ing too viscous a solution, the ester probably has .an intrinsic viscosity'too low for the preparation of yarns havin good tensile strength. v

We have found that the acetone viscosity of cellulose acetate and similar esters may be measurably reduced without degrading the cellulose by incorporating in those esters an alkalimetal acid phosphate having a pH range of 4-6 so that ,05.1% of the phosphate is present in the cellulose esters. This incorporation of theacid phosphate can easily take place in the 2 last washin step in the preparation of the cellulose ester and upon drying the product so treated, the required acid phosphate content resides therein. Cellulose esters treated in this way have good stability towards heat. E The'cellulose esters which may be treated in accordance with our invention to lower their acetone viscosity are those in which, the acyl consists predominantly of acetyl but which have been hydrolyzed so that the acetyl content, if cellulose acetate, is within'the range of 38-42%. These esters are prepared by esterifying cellulose in an esterification bath of acetic anhydride, solvent, and an acetylation catalyst, such as $111- furic acid. A satisfactory method of preparing these esters is that described and claimed in Malm Patent No. 2,097,464. After the esterification has been completed, the ester is subjected to hydrolysis so as to impart acetone solubility thereto. The ester prepared may be either a simple cellulose acetate or, if desired, a small proportion of propionyl or butyryl may be employed in the esterifying liquid, it usually being desirable that no more than 10% of acyl other than acetyl be present in the final ester. I

After the hydrolysis of the cellulose ester has been accomplished, that ester is separated from the liquid in which it is dissolved, such as by pre- 'cipitating in dilute acetic acid. The cellulose'ester is then thoroughly washed with water and dried. It is inthe last washing step of this operation that the imparting of an acid phosphate content to the cellulose ester may-be conveniently accomplished. The cellulose ester, due to the washing in the water usually available for this purpose, takes on a certain proportion of calcium or magnesium and, thus, has an alkaline earth metal residue therein. By incorporating acid phosphate in the cellulose ester the viscosity-raising effect of this residue is negatived and the cellulose ester shows substantially the same acetone viscosity as if thealkaline earth metal residue were absent therefrom.

The concentration of acid phosphate employed in the water washing step is governedby the amount of water containing. the acid phosphate which is retained by the cellulose ester before cellulose ester after drying. In other washing operations the ratio of water to cellulose. esterv in the finished productpriorto drying may' be as much as three parts of water to one part of ester. In such a case it would be desirable to use water containing a concentration of acid phosphate of .02%-.04% therein to obtain in the dried cellulose ester the desired acid phosphate content of .05-.1

In the manufacture of cellulose acetates there are many procedures which might result in introducing an alkaline earth metal residue, such as of calcium or magnesium into the cellulose ester. For instance, the esters having such a residue may have resulted from contacting with a calcium or magnesium salt during their preparation, washing, or refining. The wash water might have been water of usual hardness and contained calcium compounds such as calcium.

sulfate and calcium carbonate. Another method by which magnesium or calcium residue might have been incorporated in the cellulose ester is by neutralizing part of the sulfuric acid in the cellulose ester reaction mixture with a magnesium or calcium compound before hydrolysis as described and claimed in U. S. Patent No. 2,259,462 of Fletcher or by neutralizing the catalyst remaining after hydrolysis with magnesium or calcium compound. The ester resulting from any of the various operations described will have an alkaline earth metal residue, which residue acts to increase the viscosity of the cellulose ester beyond that which the ester would have if such a residue were not present. Analysis shows that this residue to be effective need only be present in the ester in a minute amount, such as on the order of 005%, although a larger quantity of the alkaline earth metal material in the cellulose ester will, of course, have increased viscosity-raising eifect. This residue seldom, if ever, exceeds 0.1%.

The incorporation of the acid phosphate into the cellulose ester as described herein results in a reduction of acetone viscosity of the cellulose ester ranging up to as much as 45% when compared with a cellulose ester which has not been treated to destroy the viscosity effect of this residue. The cellulose ester, however, is not broken down as shown by the fact that the intrinsic viscosity thereof is substantially the same as that of the ester which has not been so treated. Consequently, less solvent is necessary in preparing products from cellulose esters which have been acid phosphate treated and, as a result, the sheeting, yarn or other products prepared will have good tensile strength and, also, the heat stability of the ester is good. Furthermore, in the use of these cellulose esters in other situations where flowability is of importance, the use of products as obtained by the acid phosphate treatmentfis of value.

The following examples illustrate the lowering of acetone viscosity of hydrolyzed cellulose esters by treating with acid phosphate in accordance with our invention:

Example 1. 'A cellulose acetate having an acetyl content of 38.5% of the type useful for making cellulose acetate yarn made in accordance with a process wherein a calcium or magnesium residue was imparted thereto was, after hydrolyzing and washing free of acid, placed in a wash tank and washed with water in the proportion of pounds of water per pound of cellulose ester (dry weight). The water employed for washing the cellulose ester contained therein .03% of monosodium phosphate, and the pH of the liquid was adjusted to 4.8 by adding phosphoric acid and stirring until the pH appeared to be constant near that value. The ester was then allowed to soak for twenty-four hours whereupon the contents of the tank were centrifuged until the ester contained two parts of liquid per part of cellulose acetate. The cellulose acetate was then dried zfor 12-15 hours in a current of dry air. The resulting cellulose acetate was found to have an acetone viscosity 23.6% lower than the viscosity of another portion of the ester which had been washed without being treated with sodium acid phosphate. The acetone viscosity of the cellulose ester was determined by dissolving one part of the ester in four parts of acetone and determining the viscosity of the resulting solution.

Example 2.--The above procedure was repeated except that the pH employed in treating with the acid phosphate was 4.0 instead of 4.8. The viscosity obtained, compared with that of the ester to which no phosphate or other viscosity lowering treatment had been imparted, was 31.4% lower.

The products from each of the above examples were dissolved in acetone, and it was found that spinning solutions having more than 27% solids therein were readily obtained. These solutions were found to be eminently suitable for standard evaporative spinning operations.

The water in which the acid phosphate is dissolved for the incorporation of that material in the cellulose ester may be any water customarily used for washing operations in the manufacture of cellulose esters. Ordinarily the water employed for this purpose has a hardness varying from 100-450 p. p. m. and is not over 200 p. p. m. If desired, however, the water used can be one in which a considerable portion of the hardness has been removed or it can even I be distilled water.

We claim:

1. Partially hydrolyzed acetone soluble cellulose acetate having therein alkaline earth metal residue and '.05.1% of an alkaline metal acid phosphate whose solution in water has a pH of 4-6, which ester will withstand a heat of 180 C. for 2 hours without objectionable discoloration, and exhibits a viscosity in acetone unatfected by the alkaline earth metal residue.

2. Partially hydrolyzed acetone soluble cellulose acetate having therein an alkaline earth metal residue and .05.1% of a sodium acid phosphate whose solution in water has a pH of 4-6, which ester will withstand heating at 180 C. for 24 hours without objectionable discoloration, and the viscosity of which ester in solution with acetone is not affected by the presence of the alkaline earth metal residue.

3. A method of refining partially hydrolyzed acetone soluble cellulose acetate which comprises immersing the cellulose acetate in a bath of water having a hardness of -200 p. p. m. containing dissolved therein alkali metal acid phosphate whose solution in water has a pH of 4-6 in sufficient proportion to impart a content of the phosphate to the cellulose. ester of .05-.1 removing the excess water and drying the cellulose ester.

4. A method of refining partially hydrolyzed acetone soluble cellulose acetate which comprises immersing the cellulose acetate in water having a hardness of 100-200 p. p. m. containing dissolved therein sodium acid phosphate whose solution inwater hasa pH of 4-6 in suflicient proportion to impart a content of the phosphate to the cellulose ester or Jib-.196. removing the excess water andflfi ng the cellulose ester. CITED 5. Cellulose acetate having an acetyl content The l i z r f are 0! record in he of 38.5%, gin";alkaline earth metal residue therein file s pa ent: and .05-.1'% of a sodium acid. phosphate whose 5 1 UNITED PATENTS solution inwater has a pH of 4-6, which cellu- Number Name lose acetate resists the effect of heating at 180 1 634 986 Farr C. for 24 hours without objectionable discolora- 2126489 g ""7 L y g 1927 tion, and whose solution in acetone is not affected 2395421 N b 1 1938 by the viscosity-raising effect of the alkaline 1o e e earth metalresldue.

CARL J. MALM. LEO J. TANGHE. 

5. CELLULOSE ACETATE HAVING AN ACETYL CONTENT OF 38.5%, AN ALKALINE EARTH METAL RESIDUE THEREIN AND .05-1% OF A SODIUM ACID PHOSPHATE WHOSE SOLUTION IN WATER HAS A PH OF 4-6, WHICH CELLULOSE ACETATE RESISTS THE EFFECT OF HEATING AT 180* C. FOR 24 HOURS WITHOUT OBJECTIONABLE DISCOLORATION, AND WHOSE SOLUTION IN ACETONE IS NOT AFFECTED BY THE VISCOSITY-RAISING EFFECT OF THE ALKALINE EARTH METAL RESIDUE. 